1. Field of Invention
The present invention relates to a gallium nitride (GaN) -based light-emitting diode (LED), and particularly to a method of separating a substrate and a thin film thereon in a vertical GaN-based LED structure.
2. Related Art
Semiconductor light-emitting diodes (LEDs) have developed for several decades and luminous efficiency thereof has played a key role in the further application of LEDs in lighting facilities generally used in daily life. Therefore, the development and research of LEDs have been focused on improvement of luminous efficiency. However, heat issue has been a primary limitation on the luminous efficiency of the LEDs, since LEDs do not work normally without appropriately removing heat generated from the LEDs. Therefore, efforts of development and research of LEDs are actually paid to improvement of luminous efficiency and evacuation of heat.
Numerous methods of heat evacuation from an LED exist. For example, a metal-based circuit substrate replaces a poor heat-conductive printed circuit board (PCB), and thus the heat around the LED may be removed by virtue of the metal. On the other hand, evacuation of heat inside the LED structure may be also further improved. For example, sapphire is a suitable bulk material for growth of GaN-based layer in an LED, but it is a poor heat-conductive non-electricity conductive material. Consequently, sapphire is generally first used as a substrate for growth of GaN-based material and then is separated from the LED structure. Next, the LED structure with the sapphire substrate removed is bonded with a better heat-conductive substrate, which is illustrated in FIG. 1A and FIG. 1B.
Since the sapphire substrate is used only for growth of thin film layers and is removed after the thin film layers are formed, the sapphire substrate is termed here “transition substrate”. In FIG. 1A, the vertical LED structure 10 is in an unfinished stage. In the figure, a transition substrate 16 is used for growth of thin film layers and the thin film layers comprise an n-type GaN-based layer 15, an active layer 14, a p-type GaN-based layer 13 and a metal substrate 11. Additionally, a p-type electrode 17 is formed on the metal substrate 11. Since the transition substrate 16 is provided as a support for subsequent processes, the transition substrate 16 is removed after completion of the vertical LED as shown in FIG. 1B. In FIG. 1B, the vertical LED structure 10 has been rotated 180 degrees as compared to the structure in FIG. 1A. The transition substrate 16′ is first removed and then an n-type electrode 18′ is formed over the n-type GaN-based layer 15′ and the vertical LED structure 10′ is manufactured successfully.
Referring again to FIG. 1B, the formed vertical LED structure 10′ is thus named because the thin film layers in the structure has no lateral structure. The vertical structure 10′ not only improves evacuation of heat but also has a larger light-emitting area since its two electrodes are not disposed along the lateral direction of the LED structure and will not block irradiation of generated light. Reference is made to FIG. 1C, which illustrates the vertical LED structure 10″ with the transition substrate removed in FIG. 1B. A part of the light generated by the active layer 14″ is transmitted towards the p-type GaN-based layer 13″ and another part of the light generated by the active layer 14″ is transmitted towards the n-type GaN-based layer 15″, indicated by the arrows. If the side near the n-type electrode 18″ is the desired light output side, the light portion towards the p-type electrode 17″ is wasted.
Several technologies with regard to separation of a transition substrate and a thin film thereon in a vertical structure have been set forth. For example, a weak structure is fabricated between the transition substrate and the p-type semiconductor material or n-type semiconductor material. When a force is applied, the transition substrate is separated from the p-type semiconductor material or n-type semiconductor material. As an alternative example, a laser light is provided to separate the transition substrate and the thin film thereon. Since the thin film on the transition substrate absorbs the energy of the laser light, the side of the thin film adjacent to the transition substrate may be melted and separated from the transition substrate. The published patent application US20030150843 has disclosed the similar technology. Referring to FIG. 2 in the patent application, a line form laser light 23 scans a thin film 21 on a transition substrate 22 so that the interface 25 of the thin film 21 with the transition substrate 22 absorbs the energy of the laser light 23, the interface 25 adjacent to the transition substrate is melted and separated from the transition substrate 22. In scanning, the transition substrate 22 and the thin film 21 are moved together in the direction as shown. In the figure, S represents a scanning area at a scan, M represents a scanning width, and t represents a thickness of the thin film 21, where the M value is approximately equal to or smaller than the t value.
However, the above patent application still has drawbacks. For example, after the line-form light scans the thin film, melting occurs to varying degrees over the total surface of the thin film. Therefore, the transition substrate 22 and the thin film 21 may not be uniformly stripped. Further, line-form laser light may lead to a thermal stress issue, which also adversely affects separation of the transition substrate and the thin film, and the thin film is apt to crack.
In view of the shortcomings of the prior GaN-based vertical LED structure and method therefor, it is necessary to provide a GaN-based LED structure that does not waste generated light and a method of separating a substrate from a thin film thereon.